HYDROGEOLOGIC IMPLICATIONS OF OBSERVED MAJOR-ION CHEMISTRY AND ENVIRONMENTAL ISOTOPES

Examination of data from this study, in combination with work by previous investigators, suggests several possibilities to help explain the composition of samples collected from the Wasatch aquifer and the underlying coalbed aquifers in this study. All interpretations of data were provided in previous sections, along with references to previous conclusions reached by other investigators. It should be noted that interpretations presented herein are a combination of past investigations and data collected as part of this investigation. Sample size for this study was limited, and as more data are collected in other areas of the basin, these interpretations may be modified or refined.

In the previous section, two possible concepts were hypothesized to explain the observed major-ion chemistry and stable isotope values at three locations with monitoring-well clusters in the study area. The first concept proposes that the changes observed with depth at the three monitoring-well cluster locations are the result of geochemical processes that occur as ground water moves vertically through successively deeper, hydraulically connected sandstone lenses in the Wasatch Formation and finally, into the underlying coalbed aquifer. This investigation, along with earlier investigations described herein, has noted a hydraulic potential for downward ground-water flow within the Wasatch Formation. If geologic conditions are favorable to vertical ground-water flow, geochemical processes such as dissolution, precipitation, ion exchange, sulfate reduction, and mixing of waters are the processes that may occur as ground water moves downward through the Wasatch Formation and evolves the water to the sodium-bicarbonate type observed in the deeper part of the Wasatch Formation and the coalbed aquifers.

The second concept assumes the presence of two different aquifers or aquifer systems to explain the differences in major-ion chemistry and stable isotope values observed at the three monitoring-well clusters. Three ground-water samples were collected from shallow wells in this study and all three were collected from the shallow part of the Wasatch Formation (less than about 200 ft below land surface). The wells had mixed cation composition (but generally dominant in calcium and magnesium) with either sulfate or bicarbonate as the dominant anion; all three wells were located at the monitoring-well clusters previously discussed. These wells could be part of a shallow aquifer or aquifer system represented by the "shallow geochemical zone" discussed previously. All ground-water samples collected from wells completed deeper in the Wasatch Formation and the underlying coal beds were sodium-bicarbonate-type waters; these wells could be representative of the underlying, deeper, chemically stagnant geochemical system described by Lee (1981) (described herein as the "deep geochemical zone" composed of the deep sandstone lenses present in the Wasatch Formation and underlying coal beds). In this explanation, little vertical hydraulic connection is present between successively deeper sandstone lenses in the Wasatch Formation and between the shallow sandstone lenses and the underlying coalbed aquifers; very little vertical flow, and therefore, intermixing of waters between the shallow and deep geochemical zones would occur. Heterogeneity and anisotropy, related to discontinuous sandstone lenses surrounded by a predominantly fine-grained lithology present in the Wasatch Formation, could have a large effect relative to the actual ground-water flow direction and result in ground-water flow that is primarily horizontal. Other investigators such as Feathers and others (1981) and Lowry and others (1993) have suggested that there is very little vertical ground-water flow in the Wasatch and Fort Union Formations because of the predominantly fine-grained lithology, and that ground-water flow in these formations is primarily horizontal through the discontinuous sandstone lenses present. Therefore, well depth (and consequently, differences in ionic composition and stable isotope values) may simply reflect the relative distance water has flowed through the aquifer and different hydrological and geochemical origins and evolutional paths. Waters from the "shallow geochemical zone" may represent waters in local ground-water flow systems with relatively short flowpaths, whereas waters from the "deep geochemical zone" may be representative of a deep, regional ground-water flow system. This explanation also is consistent with differences in water chemistry noted in this and earlier studies, assuming very little vertical ground-water flow through the Wasatch Formation and into underlying hydrogeologic units such as the coalbed aquifers.

Both of the proposed concepts can explain the observed composition of waters in the Wasatch Formation and sodium-bicarbonate composition of waters in the coalbed aquifers at the locations examined. In addition, the concept proposed by Heffern and Coates (1999) discussed earlier also can evolve the water in the coalbed aquifers to a sodium-bicarbonate type. At the basin scale, it is possible, and perhaps most likely, that all three concepts of the ground-water system are correct - the predominant hydrogeologic and geochemical processes at any given location are probably dependant on site-specific geologic and hydrogeologic conditions. In areas where many sandstone lenses are "vertically stacked" above coal beds and the hydraulic gradient allows for downward vertical flow, ground water may move downward through the Wasatch Formation and into the underlying coal beds. In other areas where the sandstones are relatively isolated with limited hydraulic connection, vertical ground-water flow is restricted and flow is primarily horizontal. Despite the localized differences in processes, the overall net effect at the basin scale is the system currently observed. The number of locations where vertical changes in major-ion chemistry and stable isotope values were examined during this study was limited to three locations. Examination of both major-ion chemistry and stable isotope values at additional locations throughout the Powder River Basin may help to refine or alter these proposed concepts of the ground-water system.

Six of eight wells completed in the Wasatch aquifer had no post-bomb water, and two of the eight wells (W2 and W5) had concentrations suggesting a mixture of pre- and post-bomb water, although the low concentrations are suggestive of very little modern water (table 10). One of these two wells (W5) is the shallowest well completed in the Wasatch aquifer (probably the only water-table well). This well had very low concentrations of tritium, indicating that some post-bomb water may be present near the water table. Additional samples at the water table in the Wasatch aquifer should be collected to determine if some modern water is present at or near the water table at more locations in the basin. However, if there was a significant amount of areal recharge, it would be expected that post-bomb water would be distributed throughout the shallow zone of the Wasatch aquifer. The absence of post-bomb water in the shallow zone would suggest that processes responsible for recharge to the Wasatch aquifer in the Powder River Basin are probably very slow. As discussed earlier, most recharge to the coalbed aquifers is suspected to occur in or near clinker. It is possible that the majority of recharge from precipitation to the Wasatch aquifer may occur in the highly permeable clinker scattered throughout the basin; additional recharge also probably occurs from surface-water drainages in the study area. More accurate age-dating techniques and measurement of recharge rates would be required to understand recharge processes to the Wasatch aquifer.

Based on the absence of any post-bomb water in samples collected from the coalbed aquifers, it appears that ground water may be flowing very slowly away from the suspected source of recharge, the clinker (which has modern or post-bomb water). Since no tritium data has been collected from wells completed in the coalbed aquifer adjacent to and immediately downgradient of suspected recharge areas (i.e., clinker), the rate at which water enters the coalbed aquifers from its recharge areas is not known.

COMPARISON OF WATER QUALITY TO REGULATORY STANDARDS

An important purpose in studying and describing the general and physical characteristics and ionic composition of water is to determine its suitability for proposed uses. In the study area, water from wells completed in the lower Tertiary aquifers (Wasatch aquifer and coalbed aquifers) and not used in relation to resource extraction (petroleum production, coal mining, or coalbed methane production) is used primarily for public supply, domestic, or agricultural purposes (Feathers and others, 1981; Wyoming Water Development Commission, 1985; Martin and others, 1988). To aid the determination of suitability for the intended uses, results of water-quality sampling are compared to U.S. Environmental Protection Agency (USEPA) and State of Wyoming water-quality levels and standards and several additional commonly used guidelines for the proposed uses.

Public Supply and Domestic Use

The U.S. Environmental Protection Agency (1991, 1996) has established drinking-water regulations for public supplies of drinking water. The regulations specify maximum contaminant levels (MCLs) and secondary maximum contaminant levels (SMCLs). The MCLS are health-based and are legally enforceable standards while the SMCLs are nonenforceable, recommended standards. Although MCLs and SMCLs apply only to public supplies of drinking water, not individual well owners, the levels are useful to determine the suitability of water for drinking. Standards also have been developed by the State of Wyoming to evaluate ground-water quality for domestic use (Wyoming Department of Environmental Quality, 1993).

Results of ground-water-quality sampling of wells conducted as part of this study are compared to selected applicable MCLs and SMCLs in table 12 and State of Wyoming domestic standards in table 13. Dissolved solids was the constituent or characteristic with the highest percentage of exceedances. More than 50 percent of concentrations from both the Wasatch aquifer and coalbed aquifers exceeded USEPA levels and State of Wyoming standards. The ground-water sample collected from spring S1 (appendix table 1) does not exceed any levels or standards. The sample collected from spring S2 (appendix table 1) exceeds the MCL, SMCL, and Wyoming standard for both sulfate and dissolved solids.

Hardness is another water-quality characteristic commonly used to characterize the suitability of water for public-supply and domestic use. Hardness usually is characterized on the basis of four classes (Hem, 1985, p. 158-159). Hardness values calculated for ground-water samples collected from wells as part of this study are summarized by aquifer in relation to the four classes in table 14. Both ground-water samples collected from springs (appendix table 1) were very hard (130 mg/L for sample collected from spring S1 and 790 mg/L for sample collected from spring S2).

Agricultural Use

Agricultural use includes water used to irrigate crops and water given to livestock. The State of Wyoming has established standards for both uses (Wyoming Department of Environmental Quality, 1993). Ground-water samples collected from wells are compared to selected applicable State of Wyoming agricultural standards in table 15.

The ground-water sample collected from spring S1 (appendix table 1) does not exceed any Wyoming agricultural (irrigation) or livestock standards. In contrast, the ground-water sample collected from spring S2 (appendix table 1) exceeds the agricultural-use standard (irrigation) for sulfate and dissolved solids.

A classification system to evaluate the suitability of water for irrigation use was developed by the U.S. Salinity Laboratory Staff (1954). The classification system is based on two characteristics, the salinity hazard and sodium (alkali) hazard of the water. Salinity hazard is divided into four classes using the specific conductance of the water. The characteristics of the salinity-hazard classes and specific-conductance ranges are as follows:

¹µS/cm, microsiemens per centimeter at 25 degrees Celsius.

The sodium hazard also is divided into four classes using the sodium-adsorption ratio (SAR). The SAR is a dimensionless ratio that is calculated to indicate the tendency of sodium to replace calcium and magnesium in soils. The replacement of calcium and magnesium with sodium can damage the soil structure and reduce the permeability of the soil to water infiltration (Hem, 1985). However, the SAR should be used in conjunction with information about the soil characteristics and irrigation practices in the area being examined. The SAR is calculated by converting ion concentrations to meq/L and substituting into the SAR equation as follows:

SAR values can then be compared to characteristics of the four sodium-hazard classes as follows:

Typically, both salinity hazard and sodium hazard are combined into a single plot to evaluate the suitability of water for irrigation (U.S. Salinity Laboratory Staff, 1954). Results from analyses of ground-water samples collected from springs and wells (appendix table 1) are shown in figure 26. Samples collected from both springs plot in the low sodium-hazard (S1) class but plot in the medium (C2) and high (C3) classes for salinity hazard. Ground-water samples collected from the Wasatch aquifer plot in the range from low (S1) to high (S3) sodium-hazard classes and medium (C2) to very high (C4) salinity-hazard classes. Ground-water samples collected from the coalbed aquifers also plot in the range from medium (C2) to very high (C3) salinity-hazard classes, but plot in range from low (S1) to very high (S4) sodium-hazard classes. The sample that plots the highest in both the salinity- and sodium-hazard classes from all aquifers was collected from the Big George coal bed. Although this suggests that samples collected from wells completed in all aquifers plot in a wide range of both sodium and salinity-hazard classes, most samples cluster in or near the combined medium-sodium-hazard—high-salinity-hazard (S2-C3) class.

Figure 26. Suitability of water for use in irrigation based on analyses of water from springs, Wasatch aquifer, and coalbed aquifers, eastern Powder River Basin, Wyoming, 1999. Diagram modified from U.S. Salinity Laboratory Staff (1954). Data are listed in appendix table 1. (Click on image for a larger version, 176 kb)

SUMMARY

Ground-water samples collected from wells in coalbed aquifers of the Fort Union Formation and from springs and wells in overlying aquifers displayed distinct differences and trends between and within the aquifers and areally within the Powder River Basin. Data on water levels, major-ion composition, and isotopic composition of the water support the conclusion that the changes in water composition are likely the result of geochemical processes that occur as ground-water moves vertically and horizontally, as well as possible hydraulic connection between the aquifers.

Major-ion composition of samples in this study varied, primarily in relation to depth or proximity to suspected recharge areas. Ground-water samples collected from springs discharging from clinker were classified as calcium-sulfate-type and calcium-bicarbonate-type waters. Two ground-water samples collected from wells completed in the Wasatch aquifer had mixed cation composition with sulfate as the dominant anion, and one sample was a sodium-magnesium-sulfate-bicarbonate-type water; all three samples were collected from wells completed to depths less than 200 feet. Ground-water samples collected from wells completed in the coalbed aquifers and 5 of 8 wells completed in the Wasatch aquifer were all sodium-bicarbonate-type waters. All five Wasatch wells with sodium-bicarbonate-type waters were collected from wells completed at depths greater than 200 feet. Statistically and qualitatively, major-ion composition of samples from the coalbed aquifers and the overlying Wasatch aquifer was not significantly different except for two constituents, sulfate and fluoride, with sulfate showing the most striking difference. The median sulfate concentration in the Wasatch aquifer was low (130 mg/L), but concentrations were much lower in the coalbed aquifers, with more than 50 percent of the values below detection limits.

Major-ion data and water levels in monitoring-well clusters composed of wells completed in coalbed aquifers and the overlying Wasatch aquifer indicated changes in water composition that may be related to depth and possible hydraulic connection between aquifers. Water-level measurements at most of the monitoring-well clusters (4 out of 5) indicated a potential for downward ground-water flow from the Wasatch aquifer to the coalbed aquifers, with the one remaining well cluster indicating a potential for upward movement. Both increases and decreases in major ions (Ca, Mg, SO₄, Na, HCO₃) with depth and dissolved-solids concentrations were noted in the well clusters, with no consistent trend for any of the constituents except sulfate, which dramatically decreased with depth. In the Wasatch aquifer, statistically significant correlations were noted between calcium and sulfate concentrations and well depth, and correlations close to statistical significance were noted for dissolved solids, magnesium, and fluoride with increasing well depth. In the coalbed aquifers, a statistically significant correlation was found between potassium concentrations and well depth, but qualitatively, again the most striking difference was the decrease in sulfate with depth.

No clear areal pattern in water type was noted because samples collected from most wells, regardless of aquifer type, were sodium-bicarbonate-type waters. However, a pattern in dissolved-solids concentrations in waters from the coalbed aquifers was noted. Samples from this study and another recent investigation (Rice and others, 2000) suggests that dissolved-solids concentrations may be lower (less than 600 mg/L) south of the Belle Fourche River and that concentrations appear to increase northward in the Powder River Basin in Wyoming.

Tritium was used to qualitatively estimate the time of ground-water recharge. Tritium concentrations in water samples collected from two springs suggest that both were recharged after 1952 and contain modern (or post-bomb) water. Tritium concentrations in six of eight ground-water samples collected from wells completed in Wasatch aquifer overlying the coalbed aquifers suggest the water is submodern (or pre-bomb). Tritium concentrations in the remaining two wells suggest a mixture between submodern and modern water, although the low concentrations suggest that ground water in these wells has very little modern water. Tritium concentrations above pre-bomb concentrations were not detected in any wells completed in the coalbed aquifers, suggesting that ground water in the coalbed aquifers in the shallow coal beds of the Tongue River Member of the Fort Union Formation probably is submodern, and that no recharge water is likely to have reached the portions of the aquifers sampled in this study since at least the early 1950's. This suggests recharge to the Wasatch aquifer and the coalbed aquifers is probably very slow.

The d²H and d¹⁸O values from waters in this study suggest that the waters are of meteoric origin. Paired d²H and d¹⁸O values were plotted in relation to the Global Meteoric Water Line, a meteoric water line for North American continental precipitation, and a local meteoric water line constructed for the Powder River Basin (Gorody, 1999). The values plot close to all meteoric water lines, indicating that the water in ground-water samples collected during this investigation is of meteoric origin. The isotopic values suggest that the waters were recharged in a colder climate or at a cold temperature, mid-latitudes, and mid-continent. Samples do not group together based on aquifer origin; this suggests either intermixing of the waters in the aquifers or it suggests that the different aquifers are subject to similar recharge and/or evolutional paths for the water so that the difference in the d²H and d¹⁸O values is minimal.

The areal distribution of d²H was examined and an apparent break in d²H values along a northwest to southeast trend was observed. In the coalbed aquifers, all but one ground-water sample (collected from the Big George coal bed), show a pattern where the d²H values become more negative towards the center of the Powder River Basin, and values greater than an arbitrary reference value of -140 ‰ (parts per thousand or per mil) were observed near the outcrop area of the Wyodak-Anderson coal zone. The pattern in samples collected from the overlying Wasatch aquifer was reversed. The values more negative than -140 ‰ are near the outcrop area, and the values that are less negative than -140 ‰ are closer to the basin center. It is unclear if this pattern is a result of sample size, different recharge mechanisms, methanogenesis, or if the processes producing these differences are independent.

Results of ground-water-quality sampling were compared to selected USEPA and State of Wyoming regulatory and nonregulatory standards and several additional commonly used guidelines to determine the suitability of water for possible uses. The public drinking-water supply and domestic-use standards for dissolved solids were the standards most frequently exceeded in samples collected from both the Wasatch aquifer and the coalbed aquifers. The State of Wyoming agricultural-use (irrigation) standard for sulfate was exceeded in 75 percent of samples collected from the Wasatch aquifer and 8 percent of samples collected from the coalbed aquifers. The State of Wyoming agricultural-use (irrigation) standard for dissolved solids was exceeded in 25 percent of samples collected from the Wasatch aquifer and 8 percent of samples collected from the coalbed aquifers. The only State of Wyoming standard for livestock use exceeded was pH; 25 percent of samples collected from the Wasatch aquifer exceeded the standard. Water from the Wasatch aquifer ranged from soft to very hard and water from the coalbed aquifers ranged from moderately hard to very hard. Samples collected from wells completed in both the Wasatch aquifer and coalbed aquifers plotted in a wide range of both sodium- and salinity-hazard classes, but most samples clustered in or near the combined medium-sodium-hazard—high-salinity-hazard classes.

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APPENDIX

WATER-QUALITY ANALYSES

Table 1. Physical properties, general mineral characteristics, and major ions in ground-water samples collected from springs discharging from clinker and wells completed in the Wasatch aquifer and coalbed aquifers

Table 2. Radioactive and stable-isotope values for ground-water samples collected from springs discharging from clinker and wells completed in the Wasatch aquifer and coalbed aquifers

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